<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
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    <title>Document</title>
    <style>
        /* css2：伪类选择器和伪元素选择器都是一个冒号 */
        /* css3：让伪元素选择器增加了一个冒号 */
        .line::before,.line::after
        {
            content: "---";
        }
    </style>
</head>
<body>
    <h1 class="line">hello，world</h1>
    <h2 class="line">hello，world</h2>

</body>
</html>